There are two competing aspects to this type of situation.
On the one hand, robbery is by definition a crime against the person (as opposed to larceny, which is a crime against property). By definition, if an armed robber displays a weapon to the person in possession of the cash and demands the cash, he is making a threat to use the weapon if the cash is not forked over (and, possibly, even if it is). So, almost by definition, you have an immediate threat or death or serious bodily injury against a third party.
On the other hand, we know that statistically, most clerks will fork over the dough and most robbers, even those who have displayed a weapon, will then take the dough and flee, without shooting.
Given this, and given further the inescapable risk of causing injury as a result of one's own shot being fired, I'd not shoot unless I saw more. Exactly what more it would take requires, as someone already observed, that you be there.
On the one hand, robbery is by definition a crime against the person (as opposed to larceny, which is a crime against property). By definition, if an armed robber displays a weapon to the person in possession of the cash and demands the cash, he is making a threat to use the weapon if the cash is not forked over (and, possibly, even if it is). So, almost by definition, you have an immediate threat or death or serious bodily injury against a third party.
On the other hand, we know that statistically, most clerks will fork over the dough and most robbers, even those who have displayed a weapon, will then take the dough and flee, without shooting.
Given this, and given further the inescapable risk of causing injury as a result of one's own shot being fired, I'd not shoot unless I saw more. Exactly what more it would take requires, as someone already observed, that you be there.